1. Field of the Invention
The present invention relates generally to interference cancellation, and in particular, to an apparatus and method for canceling the interference of a neighbor cell in a Broadband Wireless Access (BWA) communication system based on a Multiple Input Multiple Output (MIMO) technique. An IEEE (Institute of Electrical and Electronics Engineers) 802.16e system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme as a physical layer scheme will be described as an example of the BWA communication system of an exemplary embodiment of the present invention.
2. Description of the Related Art
FIG. 1 is a diagram illustrating a cell structure of a general BWA communication system.
Referring to FIG. 1, the BWA communication system uses an inter-cell frequency reuse factor of 1 in order to enhance the frequency efficiency in a multi-cell environment. This is because a mobile station (MS) at the edge of a serving cell may suffer interferences from neighboring cells.
The BWA communication system allocates preambles of downlink (DL) frames to different subcarriers depending on their sectors. Thus, the preambles can be orthogonal to one another. However, other frames except the above preambles allocate pilots to subcarriers without discrimination between cells or sectors and therefore may suffer a signal interference from other cells or sectors. Moreover, when a frame of a signal of an interference cell uses a different permutation, a pilot of a serving cell is distorted by an interfering signal, which makes it difficult to estimate channel information from a signal of the serving cell as well as from a signal of the interference cell.
FIG. 2 is a diagram illustrating a signal reception scheme of a mobile station in a general BWA communication system.
Referring to FIG. 2, a consideration is given to the case where a mobile station (MS) 230 uses a Minimum Mean Square Error (MMSE) technique to estimate channel information. A channel matrix for a serving cell 210 becomes a 2×2 matrix, because a base station (BS) of the serving cell 210 has two TX antennas and the mobile station 230 has two RX antennas.
A conventional channel estimation scheme estimates a channel value by detecting a pilot subcarrier from an RX signal and dividing the detected value by a known pilot value. However, when the mobile station 230 is located at the edge of the serving cell 210, it receives an interfering signal (that is, an interference component) from a neighboring cell 220.
In this case, because the channel matrix becomes a 2×(N+2) matrix in which columns are larger in number than rows, it is difficult to detect a desired signal from an RX signal and cancel the interference thereof. This environment is modeled as Equation (1):
                              [                                                                                          r                    1                                    ⁡                                      (                    n                    )                                                                                                                                            r                    2                                    ⁡                                      (                    n                    )                                                                                ]                =                                            [                                                          ⁢                                                                                                                                            h                                                      s                            ,                            1                            ,                            1                                                                          ⁡                                                  (                          n                          )                                                                    ,                                                                        h                                                      s                            ,                            2                            ,                            1                                                                          ⁡                                                  (                          n                          )                                                                    ,                                                                        h                                                      i                            ,                            1                            ,                            1                                                                          ⁡                                                  (                          n                          )                                                                    ,                      …                      ⁢                                                                                          ,                                                                        h                                                      s                            ,                            K                            ,                            1                                                                          ⁡                                                  (                          n                          )                                                                                                                                                                                                                                  h                                                      s                            ,                            1                            ,                            2                                                                          ⁡                                                  (                          n                          )                                                                    ,                                                                        h                                                      s                            ,                            2                            ,                            2                                                                          ⁡                                                  (                          n                          )                                                                    ,                                                                        h                                                      i                            ,                            2                            ,                            1                                                                          ⁡                                                  (                          n                          )                                                                    ,                      …                      ⁢                                                                                          ,                                                                        h                                                      s                            ,                            K                            ,                            2                                                                          ⁡                                                  (                          n                          )                                                                                                                                ]                        [                                                  ⁢                                                                                                      x                                              s                        ,                        1                                                              ⁡                                          (                      n                      )                                                                                                                                                              x                                              s                        ,                        2                                                              ⁡                                          (                      n                      )                                                                                                                                                              x                                              i                        ,                        1                                                              ⁡                                          (                      n                      )                                                                                                                    …                                                                                                                        x                                              i                        ,                        K                                                              ⁡                                          (                      n                      )                                                                                            ]                    +                      [                                                  ⁢                                                                                n                    1                                                                                                                    n                    2                                                                        ]                                              (        1        )            where ri denotes an RX signal of the ith RX antenna, hs,m,j denotes a channel value between the mth TX antenna and the jth RX antenna, hi,n,k denotes a channel value of the nth interference cell of the kth TX antenna, xs,m denotes the mth TX signal of the serving cell, xi,k denotes a TX signal of the kth interference cell, and ni denotes a noise component.
When MIMO multiple antennas are used, pilots are generally transmitted through the 0th antenna and the 1st antenna alternately. That is, when a pilot is transmitted through the 0th antenna, a pilot is not transmitted at the subcarrier position of the 1st antenna. Likewise, when a pilot is transmitted through the 1st antenna, said pilot is not transmitted at the subcarrier position of the 0th antenna. Thus, the conventional MIMO channel estimation scheme can avoid an inter-antenna interference.
However, as described above, the neighboring cell signals act as interferences with RX pilots, thus degrading the channel estimation performance. This channel estimation performance degradation reduces the reliability of TX data acquisition. In this situation, because all the signals are mixed in the same subcarrier at the same time, an estimated channel value according to the conventional channel estimation scheme becomes different from an accurate channel value. This difference degrades the data recovery performance.
Accordingly, there is a need for an apparatus and method for canceling the interference of a neighbor cell in a Broadband Wireless Access (BWA) communication system based on a Multiple Input Multiple Output (MIMO) technique.